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- Castellani, Christopher, et al.
(författare)
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First record of Cyanobacteria in Cambrian Orsten deposits of Sweden
- 2018
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Ingår i: Palaeontology. - : Wiley. - 0031-0239. ; 61:6, s. 855-880
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Tidskriftsartikel (refereegranskat)abstract
- The Swedish Cambrian ‘Orsten’-type fossil sites have yielded diverse secondarily phosphatized three-dimensionally preserved microfossils, mainly of arthropod affinities. Similar material has also been recorded from Canada, the UK, Poland, Siberia, China and Australia. Only one other non-arthropod group, the Cycloneuralia, is commonly reported from any of these sites, leading to the general assumption that ‘Orsten’-type preservation is largely restricted to animals with a chitin-containing cuticle. We describe here secondarily phosphatized, originally unmineralized, thread-shaped fossils etched out of Cambrian ‘Orsten’-type deposits from the Agnostus pisiformis Biozone of the Alum Shale Formation in Sweden. These fossils strikingly resemble specimens previously described from Precambrian deposits, with at least two different morphotaxa identified (Siphonophycus kestron Schopf and Oscillatoriopsis longa Timofeev & Hermann) as well as the modern Oscillatoria. This leads us to interpret the new fossils as unbranched, uniseriate filamentous cyanobacteria. Our morphological investigations, combined with morphometrics, allow grouping the specimens assigned to O. longa into two size classes, suggesting an even higher diversity within the ‘Orsten’ assemblages. The lack of cyanobacterial material in any sample younger than the A. pisiformis Biozone indicates that rather drastic changes occurred in the environment, that is, substrate conditions on the Alum Shale seafloor. This coincided with a significant change in the composition of the trilobite communities and onset of the globally recognized Steptoean Positive Isotope Carbon Excursion (SPICE) in Scandinavia.
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| 3. |
- Bicknell, Russell D. C., et al.
(författare)
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Habitat and developmental constraints drove 330 million years of horseshoe crab evolution
- 2022
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Ingår i: Biological Journal of the Linnean Society. - : Oxford University Press (OUP). - 0024-4066 .- 1095-8312. ; 136:1, s. 155-172
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Tidskriftsartikel (refereegranskat)abstract
- Records of evolutionary stasis over time are central to uncovering large-scale evolutionary modes, whether by long-term gradual change or via enduring stability punctuated by rapid shifts. The key to this discussion is to identify and examine groups with long fossil records that, ideally, extend to the present day. One group often regarded as the quintessential example of stasis is Xiphosurida, the horseshoe crabs. However, when, how and, particularly, why stasis arose in xiphosurids remain fundamental, but complex, questions. Here, we explore the protracted history of fossil and living xiphosurids and demonstrate two levels of evolutionary stability: developmental stasis since at least the Pennsylvanian and shape stasis since the Late Jurassic. Furthermore, shape and diversity are punctuated by two high-disparity episodes during the Carboniferous and Triassic - transitions that coincide with forays into habitation of marginal environments. In an exception to these general patterns, body size increased gradually over this period and, thus, cannot be described under the same, often-touted, static models of evolution. Therefore, we demonstrate that evolutionary stasis can be modular and fixed within the same group at different periods and in different biological traits, while other traits experience altogether different evolutionary modes. This mosaic in the tempo and mode of evolution is not unique to Xiphosurida but likely reflects variable mechanisms acting on biological traits, for example transitions in life modes, niche occupation and major evolutionary radiations.
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